High strength ductile steel

ABSTRACT

The yield strength of aging nitrogenized low carbon cold rolled steel is raised to a level in the vicinity of 80,000 psi by rapidly heating the steel to a temperature in the range of 1,319*F. to 1,400*F. for a period of 3-60 seconds, quenching, prestraining at least 2 percent, and finally aging.

[75] Inventor:

HIGH STRENGTH DUCTILE STEEL Donald J. Bailey, Bloomfield Hills, Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

221 Filed: Dec. 2, 1974 21 Appl. N0.: 529,010

[ Dec. 23, 1975 Bosch et al. l48/l2.3

Suzuki et al. l48/l2.3

[ ABSTRACT The yield strength of aging nitrogenized low carbon cold rolled steel is raised to a level in the vicinity of [52] US. Cl. l. 148/123; 148/12 F [5|] Int. C211) 6/02; C2lD 9/46 ,000 psi by rapidly heating the steel to a tempera- [58] Field of Search 148/123, 12 F ture in the range of ,3 9 to L 00 for a period of 3-60 seconds, quenching, prestraining at least 2 [56] References Cit d percent, and finally aging.

UNITED STATES PATENTS 3,330,705 7/1967 Madrzyk et al. l48/l2 F 4 Chums 3 Drawing Figures VA N I I 80/ 8o J NlTROGENlZED STEEL AFTER GMR TREATMENT l l f: l 2 a BEFORE HEAT TREATMENT l LL] 0: m

E E 40- LU m E 0 Z UJ O i 1 l ENGINEERING STRAIN (7.)

U.S. Patent Dec. 23, 1975 Sheet 2 of 2 3,928,086

TREATED NITROGENIZED STEEL AFTER AGING AGED AT 400' F l u 60 MIN I o 10 MIN CRITICAL PRESTRAIN ENGINEERING PRESTRAIN (l) HIGH STRENGTH DUCTILE STEEL BACKGROUND OF THE INVENTION This invention relates to a method for treating low carbon nitrogenized steel to produce a material of suitable ductility having a high yield strength similar to high strength low alloy steels (HSLA) and being readily producible in markedly thinner gauges.

The need to reduce the weight of the automobile has become increasingly urgent in recent years with the need to otherwise accommodate weight increases due to additions of safety and emission control devices and to improve engine performance and fuel economy. These considerations have prompted interest in automobile structural materials having a higher strength-toweight ratio.

One group of such materials presently being considered is the family of the aforementioned high strength low alloy steels with yield strengths in the neighborhood of 80,000 psi. These steels offer an attractive combination of increased strength and acceptable formability.

The high yield strengh of the HSLA steel is developed through a controlled combination of grain refinement. precipitation hardening. and solid solution strengthening which results from the addition of titanium, vanadium, or niobium to the basic low carbon steel chemistry, and from the carefully controlled cooling in the hot strip mill in which such steels are produced. Often rare earth alloying elements are added to control the shape of the inclusions and hence to im prove the steels formability. Unlike other steels the 80 KSI HSLA steels cannot be produced in gauges thinner than 0.070 inch due to limitations in the present commercial rolling equipment. Generally, thinner steel gauges of lesser strength may be formed by cold rolling. The requirements of the strengthening mechanism of present HSLA steels as above indicated make then unavailable in cold rolled or thinner conditions with suitable ductility to form automotive components with strengths of about 80.000 psi.

The term low carbon steel as used herein is a steel containing up to 0.25 percent carbon and only residual amounts of elements other than those required for deoxidation. particularly silicon 0.6 percent or less, and manganese [.65 percent or less. The term nitrogenized steel as used herein is a low carbon steel containing nitrogen preferably in the range of 80 to 200 parts per million or 0.008 to 0.02 percent by weight.

SUMMARY OF THE INVENTION High strength steels in gauges of .050 inch or less are highly desirable for automotive application for Weight reduction objectives and it is the basic object of this invention to provide a method whereby a cold rolled nitrogcnized low carbon steel is strengthened to about an 80.000 psi yield strength or more and is not subject to the minimum gauge limitations of the HSLA steels.

In general the method is applicable to an aging nitrogenized low carbon cold rolled steel and comprises a first heat treatment followed by a stamping or forming step (prestrain) and then by a second heat treatment. The total added strength provided by the method is the sum of added strength provided by microstructural changes induced mctallurgically by the first heat treatment. a strength increment due to cold working the steel which involves forming the steel in the high work 2 hardening rate condition created by the first heat treatment and finally the strain age strengthening increment caused by subjecting the part to the second heat treatment.

The first heat treatment is central to the invention because it produces a pronounced increase in work hardening rate and the ultimate or tensile strength. This heat treatment. accordingly. provides the basis for increasing the final strength of the nitrogenized low carbon steel to 80,000 psi or more. Furthermore. the first heat treatment provides a decreased yield strength during forming. a high tensile to yield strength ratio and a good n value (slope of the natural log true stress vs. natural log true strain plot) which thus provides the steel with a degree of formability as good or better than the best HSLA steel commercially available today.

In general the first heat treatment consists of rapidly heating a sheet of an aging nitrogenized low carbon steel to a temperature in the alpha plus gamma region of the phase diagram for 3 to seconds and quenching. The sheet is then press formed to obtain a mini mum prestrain of at least 2 percent. The sheet is then aged at room temperature or at paint-curing temperatures to develop a final strength of about 80,000 psi.

DESCRIPTION OF THE DRAWINGS FIG. I is a stress-strain curve of the nitrogenized steel identified as AAN showing the effect of the first heat treatment of the method of this invention. A curve for an HSLA steel is shown for comparison.

FIG. 2 is a time temperature curve generally depicting the three steps of the invention.

FIG. 3 is a true stress-prestrain curve for the AAN nitrogenized steel showing the dependence of the aged yield strength on the prestrain step of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described in terms ofspecific embodiments. A commercial nitrogenized low carbon steel is selected which is basically an SAE l0 [0 mild steel with about four times the nitrogen content of the standard SAE l0l0 steel. Such steel is currently available, for example. from Inland Steel Company. Great Lakes Steel Company. and Bethlehem Steel Company. These steels have yield strengths in the range of 45,000 and 55.000 psi and are intended for applications where aging at paint curing temperatures (2l2392F.) can be used to develop final yield strengths of 60.000 psi through strain aging with a 2 percent plastic prestrain. The material is relatively fine grained with a ferritic grain size of ASTM 9-l l developed through controlled processing rather than by alloying. One sample AAO has a composition "/1 by weight of silicon less than 0.005. sulphur 0.027. manganese 0.55. nickel 0.02. chromium 0.006. cobalt less than 0.0l titanium less than 0.005. aluminum 0.004. copper 0.072, phosphorous 0.003. carbon 0.13. oxygen 0.0238. nitrogen 0.0l34 and the balance iron. A second sample AAN consists of silicon less than 0.005. sulphur 0.0I2. manganese 0.5. nickel 0.02, chromium 0.006. cobalt less than .0l. titanium less than 0.005. aluminum 0.005. copper 0.066. phosphorus 0.003. carbon 0.13. oxygen 0.0220. and nitrogen 0.0l 38.

As previously indicated. yield strengths of 80.000 psi or more can be developed in a stamping by means of the method of this invention which consists of the following thrce essential steps as illustrated in FIG. 2:

l. A rapid conditioning heat treatment applied to the steel prior to forming comprising heating the sheet rapidly to a temperature between its eutectoid temper ature to about 1,400F. for at least three seconds and then quenching;

2. A prestrain step in which the material is plastically deformed by stamping or the like to a strain level of 2 percent or more. preferably within about 5 days; and

3. An aging cycle to obtain the final desired strength. for example, at room temperature for 2 to 3 weeks or at about 400F. for to no minutes.

The conditioning first heat treatment is essentially a solution heat treatment designed to take advantage of the large increase in the solubility of carbon and nitrogen in ferrite near the cutectoid temperatures and to produce a controlled volume fraction of martensite or bainite upon quenching. This is readily accomplished by induction heating the steel rapidly as above indicated or into its alpha plus gamma phase region and near the critical A, temperature and holding the steel at this temperature for at least 3 seconds and then quenching in brine. oil, or water.

EXAMPLE I The sample AAN was induction heated to a temperature of about l,3l9F. and held at this temperature for a total time of 10 seconds including heating and soaking time. and immediately quenching in brine. As shown by the tensile stress strain curves of FIG. I, the yield stress is decreased about percent after treatment. The rate of work hardening and ultimate strength are markedly increased with tensile to yield ratios of two to one or greater and n values near 0.19 and total tensile elongations of about 20 percent. These values are typical of nitrogenized low carbon steels treated in this fashion, indicating superior formability for this steel after the first heat treatment, as compared to HSLA steels. A tensile curve for Van-80, a typical HSLA steel in the 80.000 psi yield range is shown for comparison. Final strength improvements are obtained by aging the treated material at room or elevated temperatures. For example, specimens yield at about 80,000 psi when deformed about 2 percent and aged for 12 days at room temperatures or for l0 minutes at about 400F. Treated specimens yield at about 100,000 psi when deformed 10 percent before aging 12 days at room temperature or [0 minutes at about 400F.

EXAMPLE I] The steel is heated by submersion in a liquid salt bath at abotit l,380F. for a total of seconds, and immediately quenched in brine. Again the yield stress is decreased and the work hardening rate, the tensile strength. and the tensile strength to yield strength ratio are increased. The treatment conditions for this method of heating were selected to give a steel of comparable ductility to that of Example I. Although the increase in work hardening rate is not as great (providing an ultimate strength of 78,500 psi as compared to 91,000 psi for Example I), it is sufficient to give a yield strength after 2 percent prestrain and aging of 80,000 psi.

EXAMPLE lll The steel is treated by submersion in a liquid salt bath at l,380F. for a total of 80 seconds, and immediately quenched in oil. Again the property changes outlined above were noted The decrease in tensile ductility is 4 not as great as in Examples l and ll with a total elongation of about 25 percent being typical (as compared to about 20 percent of the previous Examples). As before, additional strengthening is obtained by aging after forming. Similar results were obtained using the AAO steel.

As is apparent from the foregoing, realization of the 80.000 psi yield strength is dependent on the selection of a suitable heating temperature and the duration of the heat treatment in the alpha plus gamma region. the amount of prestrain and the aging treatment.

It has been determined that a 2 percent prestrain. as shown by the broken line in FIG. 3. is a practical lower limit for automotive applications. To achieve a final yield strength of 80,000 psi it has been found that a heating duration of at least three seconds by induction heating at a temperature of about l.380F. for the nitrogenized low carbon steel is operative to form austenite and to solubilize sufficient carbon in the ferrite and to, on quenching, convert sufficient austenite to martensite or bainite to develop the 80,000 psi yield strength on aging. The maximum duration of the heat treatment is determined by the ductility requirements in the automotive applications. Such applications typically require a tensile elongation of about 18 percent which limits the heat treatment duration at l,380F. to about seconds.

Adjustments in the duration of heating and in the temperatures selected for the first heat treatment can be used to produce a relatively broad spectrum of mechanical properties in a treated sheet. As shown in Example I. a single rapid heat treatment can be used to convert a nitrogenized low carbon steel into a product with a strength level, after forming and aging, comparable to the present family of HSLA steels. Because this invention is relatively simple to implement and is not subject to the minimum gauge restriction of the HSLA steel. it is viewed as an attractive alternative thereto.

in the practical application of this invention, the nitrogenized cold rolled steel of suitable gauge in the neighborhood of 0.05 inch is provided at the automobile component stamping site. A sheet of suitable size is preferably induction heated to a temperature for a time as above indicated and quenched. The sheet is then placed in a stamping die and formed to a desired configuration with a prestrain of at least 2 percent. Thereafter, the part is aged at room temperature or while it is taken through a painting cycle to develop the desired final strength. Accordingly, the process readily leads itself to the conventional operations presently in practice at automobile component stamping plants.

As previously stated, the first heat treatment requires a temperature within the alpha plus gamma region. It is preferred. to select a temperature just above the eutectic temperature since higher temperatures require lesser time periods and are thus more difficult to control and hence achieve product consistency. It is to be understood that the solubilizing effect is greater at the higher temperatures so that both temperature and heating duration must be controlled to obtain the desired yield strength and ductility. It has been found that with present control equipment an upper heating temperature of about l.400F. is practical and preferred.

In summary, the principle advantages for the nitrogenized low carbon steel treated in accordance with this invention as compared to HSLA steel are that it is less expensive, has equivalent or better formability with yield strength of 80.000 psi or more obtained by aging with a 2 percent minimum prestrain, no minimum gauge restrictions. lower press loads required for forming. superior residual ductility after strain aging. and the ability to form exterior appearance parts of moderate complexity since the initial heat treatment of this invention suppresses stretcher strain formation during forming.

The method has also been found to markedly improve the strength of low carbon steels generally, and although this invention has been described in terms of specific embodiments it is obvious that other forms may be adopted within the scope of this invention.

I claim: 1. The method of producing a high yield strength steel comprising the steps of:

heating an aging low carbon steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram for a time sufficient to dissolve a substantial proportion of the carbon into the ferrite of said steel and to form a predetermined proportion of said steel into austenite,

quenching said steel to substantially retain the solubilized carbon in solution and to transform at least the major proportion of said austenite into a form selected from the group consisting of martensite and bainite.

plastically deforming the steel an amount equivalent to at least 2 percent on the tensile stress-strain diagram,

aging said deformed steel for time sufficient to develop a desired superior yield strength.

said predetermined proportion of austenite being an amount necessary to provide the aged steel with said yield strength when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram.

2. The method of producing a high yield strength nitrogenized low carbon cold rolled steel comprising the steps of:

rapidly heating an aging low carbon nitrogenized steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram for a time sufficient to dissolve a substantial proportion of the carbon and nitrogen into the ferrite of said steel and to form a predetermined proportion of said steel into austcnite.

quenching said steel to substantially retain the solubilized carbon and nitrogen in solution and to transform at least the major proportion of said austenite into a form selected from the group consisting of martensite and bainitc.

plastic-ally deforming the steel an amount equivalent to at least 2 percent on the tensile stress-strain diagram.

aging said deformed steel for time sufficient to develop a minimum yield strength of 80,000 psi.

fill

6 said predetermined proportion of austenite being an amount necessary to provide the aged steel with the yield strength of about 80.000 psi when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram.

3. The method of producing a high yield strength low carbon cold rolled steel comprising the steps of:

rapidly heating an aging nitrogenized low carbon steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram in the range ofabout l,3l9F. to l.400F. for a time sufficient to dissolve a substantial proportion of the carbon and nitrogen into the ferrite of said steel and to form a predetermined proportion of said steel into austenite,

quenching said steel to substantially retain the solubilized carbon and nitrogen in solution and to transform at lemst the major proportion of said austenite into a form selected from the group consisting of martensite and bainite.

plastically deforming the steel an amount equivalent to at least 2 percent on the tensile stress-strain diagram. aging said deformed steel for time sufficient to develop a minimum yield strength of 80,000 psi,

said predetermined proportion of austenite being an amount necessary to provide the aged steel with the yield strength of about 80.000 psi when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram.

4. The method of producing a high yield strength nitrogenized low carbon cold rolled steel comprising the steps of:

rapidly heating an aging nitrogenized low carbon steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram in the range of about l,3 [9F. to l,400l-. for a time per iod of 3 to seconds to dissolve a substantial proportion of the carbon and nitrogen into the ferrite of said steel and to form a predetermined proportion of said steel into austenite,

quenching said steel to substantially retain the solubilized carbon in solution and to transform at least the major proportion of said austenite into a form selected from the group consisting of martensite and bainite,

plastically deforming the steel an amount equivalent to at least 2 percent on the tensile stress-strain diagram. aging said deformed steel for time sufficient to develop a minimum yield strength of 80,000 psi, said predetermined proportion of austenite being an amount necessary to provide the aged steel with the yield strength of at least about 80,000 psi when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram. 

1. THE METHOD OF PRODUCING A HIGH YIELD STRENGTH STEEL COMPRISING THE STEPS OF: HEATING AN AGING LOW CARBON STEEL TO A TEMPERATURE WITHIN THE ALPHA PLUS GAMMA REGION OF THE IRON CARBON PHASE DIAGRAM FOR A TIME SUFFICIENT TO DISSOLVE A SUBSTANTIAL PROPORTION OF THE CARBON INTO THE FERRITE OF SAID STEEL AND TO FORM A PREDETERMINED PROPORTION OF SAID STEEL INTO AUSTENITE, QUENCHING SAID STEEL TO SUBSTANTIALLY RETAIN THE SOLUBILIZED CARBON IN SOLUTION AND TO TRANSFORM AT LEAST THE MAJOR PROPORTION OF SAID AUSTENITE INTO A FORM SELECTED FROM THE GROUP CONSISTING OF MARTENSITE AND BAINITE, PLASTICALLY DEFORMING THE STEEL AN AMOUNT EQUIVALENT TO AT LEAST 2 PERCENT ON THE TENSILE STRESS-STRAIN DIAGRAM, AGING SAID DEFORMED FOR TIME SUFFICIENT TO DEVELOP A DESIRED SUPERIOR YIELD STRENGTH,
 2. The method of producing a high yield strength nitrogenized low carbon cold rolled steel comprising the steps of: rapidly heating an aging low carbon nitrogenized steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram for a time sufficient to dissolve a substantial proportion of the carbon and nitrogen into the ferrite of said steel and to form a predetermined proportion of said steel into austenite, quenching said steel to substantially retain the solubilized carbon and nitrogen in solution and to transform at least the major proportion of said austenite into a form selected from the group consisting of martensite and bainite, plastically deforming the steel an amount equivalent to at least 2 percent on the tensile stress-strain diagram, aging said deformed steel for time sufficient to develop a minimum yield strength of 80,000 psi, said predetermined proportion of austenite being an amount necessary to provide the aged steel with the yield strength of about 80,000 psi when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram.
 3. The method of producing a high yield strength low carbon cold rolled steel comprising the steps of: rapidly heating an aging nitrogenized low carbon steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram in the range of about 1,319*F. to 1,400*F. for a time sufficient to dissolve a substantial proportion of the carbon and nitrogen into the ferrite of said steel and to form a predetermined proportion of said steel into austenite, quenching said steel to substantially retain the solubilized carbon and nitrogen in solution and to transform at least the major proportion of said austenite into a form selected from the group consisting of martensite and bainite, plastically deforming the steel an amount equivalent to at least 2 percent on the tensile stress-strain diagram, aging said deformed steel for time sufficient to develop a minimum yield strength of 80,000 psi, said predetermined proportion of austenite being an amount necessary to provide the aged steel with the yield strength of about 80,000 psi when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram.
 4. The method of producing a high yield strength nitrogenized low carbon cold rolled steel comprising the steps of: rapidly heating an aging nitrogenized low carbon steel to a temperature within the alpha plus gamma region of the iron carbon phase diagram in the range of about 1,319*F. to 1,400*F. for a time period of 3 to 60 seconds to dissolve a substantial proportion of the carbon and nitrogen into the ferrite of said steel and to form a predetermined proportion of said steel into austenite, quenching said steel to substantially retain the solubilized carbon in solution and to transform at least the major proportion of said austenite into a form selected from the group consisting of martensite and bainite, plastically deforming the steel an amount equivalent to at least 2 percent on the tenSile stress-strain diagram, aging said deformed steel for time sufficient to develop a minimum yield strength of 80,000 psi, said predetermined proportion of austenite being an amount necessary to provide the aged steel with the yield strength of at least about 80,000 psi when said steel is deformed at least the equivalent of 2 percent on the tensile stress-strain diagram. 